The present invention relates to the treatment of gas streams to remove, concentrate and recover in unsable form, contaminants therefrom, particularly gas streams produced in rayon production.
Rayon is prepared by the so-called well-known viscose process. In this process, cellulose xanthate solution first is prepared from cellulose and then is spun into fibre. The preparation of the cellulose is xanthate solution requires an initial conversion of the cellulose to alkali cellulose by steeping in aqueous sodium hydroxide solution followed by pressing under high pressure to provide a gel-like alkali cellulose comprising about 34 wt % cellulose, about 15.3 wt % caustic and the remainder water. The alkali cellulose then is shredded to a crumblike material. The shredding step also serves to more uniformly distribute the caustic in the alkali cellulose crumb. The shredded crumb then is aged.
The aged alkali cellulose next is subjected to a xanthanation reaction using carbon disulfide by the so-called dry churn or other methods operating with addition of carbon disulfide at atmospheric pressure or at reduced pressure into evacuated churns containing alkali-cellulose crumb. Exothermic reaction of the alkali-cellulose and carbon disulfide in the xanthanation step usually is effected at about 32xc2x0 to 33xc2x0 C. and is usually completed in about 75 to 90 minutes. By-product formation, usually in the form of trithiocarbonate, produces a yellow coloration to the xanthanated product and is a source of hydrogen sulfide evolution during subsequent spinning of the rayon fibre. Hence it is preferred to operate in a manner to minimize such by-product formation.
The cellulose xanthate crumb resulting from the xanthanation reaction then is placed in large stirred tanks containing a dilute sodium hydroxide solution, to dissolve the cellulose xanthate into solution and to form a clear, honeylike, viscous dope known as viscose. The viscose solution is formed at low temperature to inhibit trithiocarbonate formation, which is undesirable. Ripening of the viscose solution to effect redistribution of the xanthate groups then proceeds prior to spinning. This step also produces the requisite xanthate concentration.
The ripened cellulose xanthate solution next is spun by extruding the viscose into a bath containing salt and acid. The salt usually is sodium sulfate and sulfuric acid usually is employed as the acid. The presence of the sulfuric acid in the spinning bath results in a reaction with the by-product trithiocarbonate formed in the xanthanation reaction and the formation of sulfurous gases, mainly hydrogen sulfide and carbon disulfide. These gases are currently exhausted to atmosphere resulting in costly chemical losses and hazardous emissions. Alternatively, scrubbing operations are employed to effect their removal with the resulting low concentration product being sewered.
A search of the records of the US Patent has been conducted with respect to the present invention and the following prior art has been. located as the closest relevant prior art:
U.S. Pat. No. 4,477,951, U.S. Pat. No. 4,368,078, U.S. Pat. No. 4,037,039, GB 789,691, U.S. Pat. No. 4,158,698,
The present invention is concerned with a procedure for the processing of the sulfurous gas-containing off-gas streams, comprising hydrogen sulfide and carbon disulfide, from the rayon spinning process and to recover these chemicals in a form which is reusable in the rayon-forming process or saleable as a commodity. While the invention is particularly described with reference to the sulfurous by-product gas stream from the rayon-making process, the invention also is applicable to other similarly-compositioned sulfurous gas streams from other industries producing a hydrogen sulfide-containing gas stream. The invention is specifically described herein with respect to the processing of the off-gas stream from the rayon spinning tank, but the principles embodied therein apply equally to other off-gas streams.
In accordance with one aspect of the present invention, there is provided a method of processing a sulfurous gas stream comprising hydrogen sulfide and carbon disulfide, which comprises contacting said gas stream with an aqueous solution of sodium hydroxide to dissolve said hydrogen sulfide and a portion of said carbon disulfide therefrom and to form an aqueous solution of sodium sulfide having carbon disulfide dissolved therein and a residual gas stream containing carbon disulfide, heating said aqueous solution of sodium sulfide above a temperature where carbon disulfide separates from said solution to form gaseous carbon disulfide and an aqueous solution of sodium sulfide depleted of carbon disulfide, recovering the gaseous carbon disulfide, concentrating the resulting aqueous solution of sodium sulfide and adjusting the pH of the concentrated sodium sulfide solution so that the sodium sulfide substantially is present in the form of sodium bisulfide, and recovering the resulting concentrated aqueous solution of sodium disulfide.